Presbyopia affects virtually every person over the age of 44. According to Jobson Optical Database, 93% of people 45 and over are presbyopic. Presbyopia entails the progressive loss of amplitude of accommodation that occurs with aging. Adler's Physiology of the Eye, which is incorporated herein by reference, discloses that the human accommodative amplitude declines with age such that accommodation is substantially eliminated by the age of 50 to 55. Accommodative ability, as defined by U.S. Pat. No. 5,459,133 to Neufeld and incorporated in its entirety herein by reference for background information, is the capacity of the eye to focus for near vision by changing the shape of the lens to become more convex.
The ocular tissues involved in the accommodative response include the lens, the zonules, the lens capsule, and the ciliary muscle. Of these, the lens is the central tissue. These structures function together to enable the eye to focus on close objects by changing the shape of the lens. The lens is centrally suspended between the anterior and posterior chambers behind the pupillary opening of the iris. The lens is supported by an array of radially oriented zonular fibers, which extend from the lateral edges of the lens to the inner border of the circumferential ciliary muscle. The ciliary muscle is attached to the scleral coat of the eye. When the eye is at rest, it is focused for distance and the lens is in a somewhat flattened or less convex position. This shape is due to the tension that is exerted on the lens periphery by the zonules. The zonules pull the edges of the lens toward the ciliary body.
During accommodation, the shape of the lens becomes more convex through contraction of the ciliary muscle, which allows the ciliary attachment of the zonules to move toward the lens, reducing the tension in the anterior zonules. This reduction in tension allows the central region of the lens to increase in convexity, thereby enabling near objects to be imaged on the retina. The processes involving the coordinated effort of the lens, zonules, ciliary body, medial rectus muscles and iris, among others, that leads to the ability of the eyes to clearly focus near on the retina is the accommodative process.
Several theories have been advanced to explain the loss of accommodation with age. These theories include the hardening of the lens with age, loss of strength in the ciliary muscle, factors related to the physical growth of the lens, and, the loss of elasticity of the lens capsule. As for the loss of strength of the ciliary muscle, it is noted that although there are age-related morphological changes that occur, there is little evidence of diminishing strength of the ciliary muscle. In fact, under the influence of pilocarpine, the ciliary muscle will vigorously contract even in presbyopic eyes.
The lens grows throughout one's life and theories have been proposed that it is this increase in size that prohibits the effects of the zonules from affecting a change in the shape of the lens. Recent works exploring this possibility have not met widespread acceptance thus far. Most of the growth of the lens is not in its diameter, but instead, in its anterior-posterior dimensions.
As for changes in the lens capsule, it has been postulated that reduction in the elasticity of the capsule is, in fact, a contributing factor in presbyopia. Moreover, it has been found that Young's modulus of elasticity for the lens capsule decreases by nearly 50% from youth to age 60, while accommodation decreases by 98%. Consequently, the principal cause of presbyopia is now considered to be “lenticular sclerosis” or the hardening of the lens.
A cataract is a condition in which the lens becomes less clear. The study of cataracts lends insight into lens and capsular changes. The usual senile cataract is relatively discus-shaped when removed from the eye, its shape being dictated by the firm lens substance. The liquefied hypermature cataract is globular when extracted, rounded up by the elastic lens capsule. This is indirect evidence that it may be possible to reverse the lenticular changes associated with presbyopia, and that the lens capsule is still sufficiently elastic.
At the present time, common treatments for presbyopia include reading glasses, bifocal glasses, or mono-vision contact lenses. All of these solutions necessitate the use of an appliance creating additional shortcomings.
Alternative theories for treating presbyopia include scleral expansion and corneal reshaping. The efficacy of such techniques is not well-established and, importantly, these techniques do not attempt to reverse what the inventors of the subject-application believe to be a substantial causation, as explained more fully below, in the loss of the accommodative amplitude of the lens typically associated with the normal aging process. Moreover, because scleral expansion and corneal reshaping involve macroscopic changes in the morphology of the lens and/or cornea it fails to reverse presbyopia.
Finally, the use of the excimer laser for the purposes of corneal reshaping to produce a multifocal refracting surface has been disclosed in U.S. Pat. No. 5,395,356. While this method seems promising, it still requires structural changes to the cornea to compensate for aging changes in the lens. Rather than trying to undo the changes brought on by presbyopia, techniques such as these merely compensate for the loss of accommodative function by altering another ocular structure.